The present invention relates generally to rotary recording mediums and to reproducing apparatus therefor.
More particularly, the invention relates to a rotary recording medium of the type wherein three kinds of reference signals are used for controlling tracking of a reproducing tracing element. The reference signals are recorded on opposite sides of a continuous spiral track, to enable a reproducing transducer to perform a movement control by following one of the above reference signals, without the use of special control signals. A reproducing apparatus is controlled by these reference signals for reproducing the rotary recording medium.
Previously, a system records an information signal as a variation of a geometrical shape along a spiral track on a rotary recording medium. Pick up tracking does not require a guide groove for a reproducing stylus, described in U.S. Pat. No. 4,331,976 granted May 25, 1982 and entitled "High Density Recording System Using Side-By-Side Information And Servo Tracks", and U.S. Pat. No. 4,315,283 granted Feb. 9, 1982 and having the same title.
On the rotary recording medium (referred to as "disc" hereinafter) is recorded an information signal such as a television video signal. Also recorded on the disc are first and second pilot or reference signals for tracking control, and a third pilot or reference signal for switching between the first and second reference signals at the time of reproduction. The first and second reference signals are alternately recorded at positions intermediate centerlines of adjacent information signal tracks. The third reference signal is recorded at a predetermined position on every information signal track.
A reproducing system uses a single reproducing element to reproduce the information signal together with the third reference, and the first and second reference signals. A tracking control signal is produced from the first and second reference signals and is switched responsive to the third reference signal separated from the reproduced information signal. The tracking of the reproducing tracing element is controlled by this tracking control signal. The reproducing tracing element traces accurately along the information signal track, even without a guide groove.
In a disc of this character, the track pitch is made very small, for example 1.4 .mu.m, in order to obtain a high density recording. For this reason, in the recording system, a high precision is required to control the pickup feed, along the disc radial direction. A light beam is used for recording the information signal on the disc. Accordingly, it is very difficult to form an unrecorded portion having a large width at the leading part, in the vicinity of the outer periphery of the disc, as in a conventional audio record disc.
Therefore, in a disc of this character, the track is formed with the normal pitch up to the outer periphery of the disc. It will be supposed that the reproduction of the information signal is to commence within 5 seconds, for example, after the instant when the reproducing tracing element is lowered onto the outer periphery of the disc. For this purpose, in the case where a video signal of 4 fields is recorded for every revolution of the disc, it is necessary that the reproducing tracing element be lowered onto the outer periphery of the disc within 105 .mu.m of a predetermined place (calculated from 1.4.times.14.times.5=105). The disc is rotated at a rotational speed of 900 revolutions per minute. Therefore, as a matter of mechanical precision, it is extremely difficult to lower the reproducing tracing element accurately onto the disc in this manner within the very narrow allowable position range in the order of merely 105 .mu.m, at the start of reproduction of the disc.
On the other hand, if the allowable range for the descent of the reproducing tracing element is a large value, a long time period is required from the lowering of the reproducing tracing element to the instant when the information signal is reached and the reproduction of the information signal begins.
It is desirable for the allowable range for descent of the reproducing tracing element to be large. Moreover, there is a short time interval from the instant at which the reproducing tracing element is lowered to the instant when reproducing of the information signal starts. The realization of both of these features at the same time has been desired in the art.
Furthermore, it is desirable to automatically return the reproducing transducer to a resting position other than a position over the disc, when the reproducing tracing element finishes reproduction to the innermost periphery of the disc. The start of the next reproduction begins from this resting position.
As one method of realizing these features, the application of the random-access method appears to be possible. More specifically, the method comprises recording beforehand an address signal over the entire track, inclusive of the track portion in the vicinity of the outer periphery of the disc. The reproducing apparatus reads out the difference between the address number of the track turn on which the reproducing tracing element was lowered and the address number of the initial track turn of the information signal, and then quickly displaces the reproducing tracing element until the address number difference becomes zero. Moreover, an operation is performed upon read-out of the final address number, in which the reproducing transducer is returned to the resting position.
By this method, however, an address signal must be recorded beforehand on the disc, and the recording system tends to become complicated. Furthermore, circuits for carrying out address detection and random access control become necessary in the reproducing apparatus, so that the reproducing apparatus becomes unduly complicated and expensive.
Furthermore, another construction can be considered in which the recording is performed without the synchronizing signal of the video signal in the innermost peripheral part of the disc. The existence or non-existence of this synchronizing signal is detected in the reproducing apparatus. In this arrangement, the reproduction transducer is returned to the resting position when the reproducing apparatus detects the non-existence of the synchronizing signal. However, since a PCM disc on which the audio signal is pulse code modulated and recorded, has no synchronizing signal, the above described reproducing apparatus detects the non-existence of the synchronizing signal and returns the reproducing transducer to the resting position immediately upon the start of the reproduction. Accordingly, there is a disadvantage in that the PCM disc cannot be reproduced by the reproducing apparatus of the above described construction.